Exploring New Advances In Internal Fixation
Prior to the broad adoption of the principles and techniques of the AO/ASIF group, cerclage wires, K-wires and Steinmann pins as well as a variety of staples were the more common internal fixation devices employed for stabilizing fractures, osteotomies and fusions. Rigid internal compression fixation techniques eventually became more commonplace and the application of these techniques to foot and ankle surgery has led to clinical advances with improved surgical outcomes. As technology advances and we increase our knowledge of bone healing from a variety of perspectives, newer designs in internal fixation devices have emerged. Some of the more recently designed fixation devices show particular promise in the field of foot and ankle surgery. With this in mind, we take a closer look at emerging two-component compression screws, locking plates and compression staples. How The New Screws Facilitate Better Compression A new generation of compression screws has arrived. These screws are two component devices that allow one to advance the head of the screw independently down the shaft of the screw to obtain additional compression. These screw designs are cannulated to facilitate placement and insertion. If they desire, surgeons may completely bury the head of the screw in bone after they have inserted the device. Examples of these devices include the Kompressor™ screw (KMI), the I.CO.S Ideal Compression Screw (Integra Lifesciences Corp.) and the Charlotte™ High Demand Compression Screw (Wright Medical Technology). We have had significant experience with both the Kompressor™ and the I.CO.S screws. The I.CO.S screw and the newly released Charlotte™ screw are designed to provide both static and dynamic compression at the osteotomy, fracture or arthrodesis interfaces. The pitch of the head of the screw is smaller than the leading threads at the distal end of the screw, resulting in static compression across the interfaces when the screw is inserted with the standard technique. The mating pitch on the shaft of the screw is actually the same as the leading threads. When the head of the screw is advanced over the shaft, one can then achieve additional dynamic compression. The basic design of the Kompressor device is similar to that of the I.CO.S and Charlotte screws. However, it is not designed to provide static compression. The pitch of the threaded portion of the trailing head of the screw is identical to the pitch of the leading threads. When the screw is inserted into the bone, no compression at the interface occurs other than what one has previously imparted to the site with a mechanical device. In this instance, one can still achieve stability and rigidity of the interface but there will be no compression. The mating thread is actually a “triple lead” thread with a much smaller pitch. This allows each complete revolution of the head of the screw a much faster advancement on the shaft than within the bone, resulting in a significant increase in compression across the interface site. Originally designed for scaphoid fractures of the hand, the Kompressor screw has many applications in foot and ankle surgery including a variety of osteotomies and tarsal bone fractures, including navicular fractures and fifth metatarsal base fractures. The screws are manufactured in two different sizes, a Mini (2.8 mm) and Standard (4.0 mm). The I.CO.S screws are available in 4.0 mm and 6.5 mm sizes. Our experience has been limited to the larger I.CO.S screw, which appears to be ideally suited for major rearfoot fusions such as the subtalar and talonavicular joints as well as calcaneal osteotomies. The technique of inserting the screw is more involved than inserting traditional cannulated and non-cannulated screws. Accordingly, there is a slightly higher learning curve and an increased demand for precise techniques. A more detailed understanding and knowledge of the anatomy of the screw, its mechanism of action and instrumentation are necessary to ensure minimal intraoperative or postoperative complications. The newer compression screws offer advantages that include minimal to no screw head prominence and less screw loosening because the entire screw (head and body) is embedded in bone within the near and far cortices. Limited in vivo testing suggests the I.CO.S screw provides better compression than traditional large cancellous bone screws. From a clinical perspective, we have observed enhanced consolidation of our fusions and osteotomies of the first metatarsal. These clinical observations have further piqued our interest and use of these devices. While removal would be seemingly difficult, we have not found this to be the case in our limited experience. The significantly increased cost of these devices may also be a consideration in today’s healthcare environment. What You Should Know About Emerging Locking Plates Traditional techniques of internal fixation with plates and screws have focused on providing absolute rigidity and stability after anatomic reduction to ensure predictable primary bone healing. There are numerous applications in foot and ankle surgery. Impaired bone healing is an unfortunate complication of internal fixation, even with plates and screws. It has been suggested that disruption of cortical vascular supply caused by direct compression of the periosteum by the plate itself, loss of reduction in osteoporotic bone or cases of hardware failure are all factors that likely contribute in varying degrees to bone healing complications such as delayed unions and non-unions. Locked internal fixation devices such as a plate with locking screws offer an innovative solution. They are designed to provide enhanced rigidity and stability to the bone site. The concept of locking plates and screws has been practiced for decades in the areas of oral maxillofacial surgery and spinal fusions. It is relatively new to extremity surgery, especially surgery of the foot and ankle. Existing systems include LCP™ Locking Compression Plates (Synthes), VueLock® Anterior Cervical Plating System (EBI) and Peri-Loc™ (Smith and Nephew). Conventional plating serves to resist bending, torsional and axial loading by transferring these loads to the bone-plate interface in the form of shear stresses that friction forces resist between the plate and the bone. Standard fixation methods rely on plate to bone compression to maintain stability and require rather high screw torque to resist motion. If the applied axial forces exceed the frictional force present at the plate-bone interface, one will have compromised stability at the fracture site. This may lead to failure and a variety of subsequent bone healing complications may ensue. Locking plates and screws create a “single-beam construct” by eliminating motion between the beam components, specifically the plate, screws and bone. One would drill and insert the locked screw at a fixed angle in order to engage the corresponding threads of the plate with the threaded screw head. This allows the locked plate to function as a fixed-angle device. It is similar to the principles of external fixation in that axial stability and pullout strength are determined by the sum of all locked screws instead of a single screw as is the case with traditional plating techniques. This significantly reduces the risk of secondary loss of reduction due to increased load. Since locked plates do not rely on frictional forces for stability, direct plate to bone compression is not required so one can preserve periosteal blood supply. The locked plates also offer additional stability in cases of osteoporotic bone or comminuted fractures in which the compromised bone may not be able to resist the high screw torque required for stability in conventional techniques. The use of locking plates and screws provides significant improvements in the overall rigidity of the surgical construct. This combination is less likely to see migration of screws and failure of the plate itself. However, further clinical studies are still needed and no doubt will be forthcoming in the near future. These plates allow one to utilize both conventional plating techniques as well as those techniques that lock the screw to the plate. When performing open reduction internal fixation of fractures, interfragmentary screw compression is still required prior to applying the locked plate as a buttress. The technique is more exacting. Therefore, the surgeon must be knowledgeable of all facets of the instrumentation and insertion techniques to ensure successful and uneventful application without surgical misadventure. Assessing The Potential Benefits Of New Compression Staples Although screw fixation has been successful in clinical use, it commonly requires modification to traditional surgical techniques including: alteration and modification of osteotomy design; the insertion of significant implant material; and the use of fluoroscopy for placing the screw(s). Some would consider this contrary to the principles of minimally invasive surgery. Recently, new technology has developed staples that are significantly improved over the traditional bone staples. The staples, which are promoted as providing both reduction and compression, include the UNI-CLIP® (Newdeal), the Memodyn Staple (Bio Research Innovations) and the OSStaple™ (BioMedical Enterprises). Other staples are also available in Europe with similar design and material. In theory, these devices not only stabilize the fusion, osteotomy or fracture site but also impart mechanical compression to facilitate rigidity and, presumably, clinical and radiographic bone healing processes. The ability of these new staples to impart compression allows the surgeon to return to traditional operative techniques while maximizing the stability and rigidity of the healing osseous interface areas. We have formally attempted to achieve the same outcome by inserting non-compressive staples after establishing the pre-compression via an external distraction/compression device. We utilize conventional staples, such as the Smith and Nephew Standard Staple (Smith and Nephew) to maintain compression exerted by an external device such as the Synthes Small Distractor (Synthes), the Tarsal Joint Distractor (Orthovation) or the Weinraub Joint and Calcaneal Spreader (Innomed). Anecdotally, we believe the standard staples can not only stabilize the fusion or osteotomy site but also maintain the compression created by a temporarily applied external device. One can mechanically actuate the UNI-CLIP with a pliers-type of instrument. This staple has two legs and a diamond shaped bridge. Using the pliers, the surgeon bends the diamond-shaped metal bridge of the UNI-CLIP to close the legs of the staple. Since the metal used in the UNI-CLIP has both elastic and plastic deformation properties, one can anticipate the staple to close, providing measurable compression force directly related to the strength of the surgeon actuating the pliers. In contrast, the OSStaple and the Memodyn Staple are capable of providing compression across an interface without the need of an external compression device. These staples are composed of NiTinol (nickel-titanium alloy) that imparts unique material thermoplastic properties. At high temperature, the metal is transformed and causes the implant to return to a pre-programmed shape. After inserting the staple, one can heat the implant with electrical current to bring the staple legs closer together. This movement results in the application of residual dynamic compression forces to the healing interface. With good physician controlled reduction and less than a full shape change of these staples, one can facilitate the storing of energy in the implant to impart residual compression to the bone healing interface. Like the other devices in this article, the cost of these “new generation” staples is significantly higher than more traditional conventional staples. While the technique of insertion is not significantly different from that of other conventional staples, it is more exact and precise. The variety of sizes (leg length, bridge width and thickness of the implant) permit wide application of these staples to a variety of osteotomies and fusions routinely performed in foot surgeries. We have used them very successfully with consistent results in places where one might normally consider use of a plate and/or multiple screws. The amount of compression achieved with these “compression” staples is subject to debate. No data has been published to date. We have recently completed a bench study to objectively measure differences in the amount of compression imparted by using both mechanical and thermodynamic staples. This information will be forthcoming in the near future. In Conclusion As one can appreciate, significant advances have emerged in the area of internal fixation for stabilizing a variety of fractures, fusions and osteotomies in extremity surgery. While further clinical studies and bench testing are needed to justify their widespread use given their increased cost and technique of application, they show great promise based on our experiences to date. We hope this article has increased the awareness of these devices and their potential use in foot and ankle surgery to enhance clinical outcomes for the benefit of our patients. Dr. Yu is the Director of the Podiatric Surgical Residency Program (PSR-36) at the St. Vincent Charity Hospital in Cleveland. He is a Fellow of the American College of Foot and Ankle Surgeons, and is a Diplomate of the American Board of Podiatric Surgery. Dr. Yu is also the Director of Program Development and a faculty member of the Podiatry Institute. Dr. Schinke is a third-year resident within the Podiatric Surgical Residency Program at the St. Vincent Charity Hospital in Cleveland. Drs. Meszaros and Shibuya are second-year residents at the aforementioned facility.
References 1. Hintermann, M.D, Valderrabano, M.D., Nigg Dr sc nat. “Influence of Screw Type on Obtained Contact Area and Contact Force in a Cadaveric Subtalar Arthrodesis Model.” Foot and Ankle International. Vol 23, No. 11: November 2002. 2. Egol, M.D, Kubiak M.D., Fulkerson M.D., Kummer PhD, Koval M.D. “Biomechanics of Locked Plates and Screws. Journal of Orthopedic Trauma. Vol 18, No. 8: September 2004.